Whistle pulls a guard toward you so you can assassinate him out of sight of his friends. It is in your Stealth kit from Sequence 1.
Whistle sits at position 1 of 19 in the Stealth tree. Whistle costs no skill points. The twins learn it during Sequence 1. Jacob and Evie each have their own skill tree, so you buy Whistle once for each twin.
Air Assassination
Drop onto a target from above and kill him in a high-profile strike.
Assassin Steps
Greatly reduces fall damage, silences your landings and makes your movement much quieter.
Chameleon
In Sneak Mode, Evie becomes invisible while standing still; only enemies very close by can spot her.
Double Assassination
Kill two nearby unaware enemies at the same time.